The invention relates generally to imaging systems, and particularly relates to illumination systems for use in imaging systems.
Conventional imaging systems typically include an illumination system for generating a field of illumination, and an optical assembly for applying the field of illumination in a modulated form to an imaging surface. Such illumination systems may provide a line of laser illumination so that a line of picture elements (or pixels) may be imaged at a time for efficiency in imaging. The field of illumination may be modulated by selectively controlling the illumination system (e.g., as disclosed in U.S. Pat. No. 4,804,975), or by using a light modulator for selectively modulating the field of illumination. Illumination systems that modulate the illumination field generally require that relatively high powers be switched on and off at fairly high speeds. This may be relatively expensive and difficult to use to achieve high quality and/or high resolution imaging. It is desirable, therefore, that light modulators be used in certain applications. The use of light modulators permits the illumination system to provide a relatively uniform field of illumination. This allows the laser emitters to exhibit relatively uniform power consumption and be maintained at a relatively uniform temperature, which also contributes to uniformity of the illumination field.
Illuminations systems for providing uniform fields of high power illumination typically include an array of laser diode emitters and a variety of optical elements that are positioned to adjust the size and uniformity of the field of illumination, and to direct the illumination field to a light modulator. U.S. Pat. No. 4,786,918, for example, discloses a laser diode array that includes a plurality of spatially disposed laser emitters that may be used for printing with a laser line modulator. A light modulator for conventional systems may either transmit the modulated illumination field through the light modulator toward the imaging surface (e.g., as disclosed in U.S. Pat. No. 5,521,748), or may reflect the modulated illumination field toward the imaging surface (e.g., as also disclosed in U.S. Pat. No. 5,521,748 as well as U.S. Pat. Nos. 5,132,723 and 5,049,901).
Conventional imaging systems that include a light modulator typically focus the near field image of the light source onto the light modulator. For example, the systems disclosed in U.S. Pat. No. 5,521,748 each provide that the near field image of the light source is focused in the slow axis direction onto the light modulator by imaging optics. Near field imaging onto the modulator, however, requires that the physical arrangement of the light source, the imaging lenses and the modulator be very accurately positioned to ensure that the focal point for the light from each of the emitters be located on the modulator. If the focal point is not located on the light modulator, then the quality of the image may be comprised. Also, if the focal length is rather short, then divergence becomes a significant concern, since divergence is inversely related to the focal length of a lens. Moreover, small variations in the arrangement and/or power of the various emitters vis-à-vis each other may result in significant variations in the light field imaged onto the modulator.
Illumination systems as disclosed in U.S. Pat. No. 5,521,748 include an array of emitters and an array of lenslets located adjacent the emitters such that specific portions of the light modulator are each illuminated by a specific emitter and its associated lenslet. Such illumination systems may provide non-uniform illumination fields on the light modulator due, at least in part, to variations among emitter characteristics and/or any overlap of illumination portions at the modulator that may occur in an effort to ensure complete coverage of the modulator.
As discussed in U.S. Pat. No. 5,900,981 a type of non-uniformity in an illumination field that may result from the use of a laser diode elements is the smile effect in which the center portion of the illumination line sags, missing the active portion of the modulator. The systems disclosed in U.S. Pat. No. 5,900,981 is disclosed to utilize natural aberrations and/or artificial aberrations for the stated purpose of decreasing the sensitivity of the optical system to the smile phenomenon. The systems of U.S. Pat. No. 5,900,981, however, also provide that the near field image of the light source is focused in the slow axis direction onto the light modulator by imaging optics. Such systems may suffer from many of the shortcomings discussed above. Near field imaging, for example, does not permit the imaging system to be telecentric in the slow axis, allowing the distance between slow axis optical elements to be changed without affecting illumination quality. Telecentricity occurs when rays are normal to an incident surface.
Illumination systems for use in thermal imaging systems in which a thermal recording medium is imaged, typically require greater power than that required in light sensitive imaging systems, since at least a portion of the recording medium must be thermally ablated during imaging. If the illumination source is comprised of a plurality of laser diodes, thermal imaging systems generally require that the fill factor (width of each emitter/spacing of the emitters) be increased. Increasing the fill factor, however, generally causes divergence to increase as well. Divergence relates to the widening of a light field as it travels away from a source.
It is an object of the present invention to efficiently provide an illumination system for imaging systems that efficiently produces a uniform illumination area at a light modulator.
It is another object of the present invention to provide such an illumination system that exhibits minimal illumination divergence.
It is a further object of the present invention to provide such an illumination system that exhibits telecentricity to facilitate magnification and/or focusing adjustments during use.
It is also an object of the present invention to provide an illumination system with a relatively high fill factor, yet relatively low divergence.
It is also an object of the present invention to provide an illumination system that achieves the benefits of near field imaging (e.g., magnification) and far field imaging (e.g., less divergence) in one system.
The invention provides an improved illumination system for use in imaging systems that may produce a non-overlapped near field image in the slow axis direction, and a far field image of the illumination source in the slow axis direction at a light modulator. In an embodiment, the illumination system produces an area of illumination for a light modulator along a slow axis direction and along a fast axis direction, and includes a plurality of laser diode emitters, a first array of first micro lenses, and a second array of second micro lenses. The plurality of laser diode emitters are arranged in an array, and each of the laser diode emitters produces illumination in a slow axis direction and in a fast axis direction. Each first micro lens in the first array corresponds to one of the laser diode emitters, and collimates illumination in the slow axis direction. Each of the second micro lenses in the second array corresponds to one of the first micro lenses. Each of the second micro lenses is arranged to receive illumination from one of the first micro lenses. The first micro lenses produce a non-overlapped near field image in the slow axis direction.